Probing the atmospheres of extrasolar planets

The HR 8799 system as seen by the Keck Telescope in 2008. The central messy region is where the star has been masked out.

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Ok, so you’re young, you’re surprisingly dusty, and you don’t match the models. No, not a picture of my geeky childhood, but the extrasolar planet HR 8799b. It orbits the star HR 8799 and, along with its two companions, is one of the two extrasolar planetary systems to be directly imaged, as shown above. Unsurprisingly it’s the dot labelled ‘b’.

If directly imaging extrasolar planets is hard (because they are so much fainter than their host stars), then taking spectra of them to study the composition of their atmospheres is, well, harder. This is just what astronomers at the University of Hawaii have managed to do for HR 8799b however, using the adaptive optics system on the Keck Telescope. To quote Trent Dupuy, one of the co-authors on the paper (from their handy press release),

Adaptive optics systems on Keck and other large ground-based telescopes make sharper images than even the Hubble Space Telescope. With adaptive optics, we are learning an incredible amount about objects that are smaller than the lowest-mass stars and larger than the most massive gas-giant planets in our solar system.

When they analysed their data they found little or no methane, which, since the amount of this gas can be used as a thermometer, meant that the planet is ~1200 Kelvin; this is ~400 Kelvin warmer than its predicted to be by the current best theoretical models. This, they think, is because it has more dust and clouds in its atmosphere than expected.

In related news, in a totally different part of the Galaxy, two more groups of astronomers (in Exeter and Florida) have also been taking spectra of two other extrasolar planets – HD 80606 b and XO-2b, this time using the Gran Telescopio Canarias. In these cases however, their big result is the first detection of potassium in their atmospheres, a result that this time was just what the modellers were expecting.

This is an exciting time for exoplanet work. The data are finally catching up with the models and, as usual some things match well, and some things raise more questions than they answer. Clearly, there are fun times ahead!